1. Field of the Invention
The present invention relates to a night vision system, and more particularly to an improved photocathode for use in a night vision image intensifier tube.
2. Description of the Related Art
Night vision systems are commonly used by military and law enforcement personnel for conducting operations in low light or night conditions. Night vision systems are also used to assist pilots of helicopters or airplanes in flying at night.
A night vision system converts the available low intensity ambient light to a visible image. These systems require some residual light, such as moon or star light, in which to operate. The ambient light is intensified by the night vision scope to produce an output image which is visible to the human eye. The present generation of night vision scopes utilize image intensification technologies to intensify the low level of visible light and also make visible the light from the infra-red spectrum. The image intensification process involves conversion of the received ambient light into electron patterns and projection of the electron patterns onto a phosphor screen for conversion of the electron patterns into light visible to the observer. This visible light is then viewed by the operator through a lens provided in the eyepiece of the system.
The typical night vision system has an optics portion and a control portion. The optics portion comprises lenses for focusing on the desired target, and an image intensifier tube. The image intensifier tube performs the image intensification process described above, and comprises a photocathode to convert the light energy into electron patterns, a micro channel plate to multiply the electrons, a phosphor screen to convert the electron patterns into light, and a fiber optic transfer window to invert the image. The control portion comprises the electronic circuitry necessary for controlling and powering the optical portion of the night vision system.
The limiting factor of the image intensification tube is the photocathode. The most advanced photocathodes are the third generation, or Gen 3 tubes, which have a long wavelength spectral response cut-off which corresponds to light having a wavelength of 940 nanometers. Thus, infra-red light having wavelengths above that range cannot be seen using the Gen 3 tube. Since there is an abundance of night sky radiation in the longer wavelengths, and various ground elements, such as foliage, have high reflectance at those wavelengths, it would be desirable for a night vision system to be able to receive those wavelengths. In addition, laser beams used by potentially hostile forces for targeting purposes operate at wavelengths of 1060 nanometers, and it would be particularly desirable for a night vision system to be able to detect these laser beams.
It has long been hypothesized by those skilled in the art that a photocathode having an indium-gallium-arsenide (InGaAs) active layer would provide the desired response characteristics. To date, InGaAs had only been used in the reflection mode and not in the transmission mode. Reflection mode refers to a usage of a semiconductor photocathode material in which electrons are emitted from a surface of the semiconductor in response to light energy striking the same surface. Reflection mode usage is typical in semiconductor cathodes housed inside vacuum tubes. Transmission mode refers to a usage of a semiconductor photocathode in which light energy strikes a first surface and electrons are emitted from an opposite surface. Photocathodes as used in modern night vision systems operate in the transmission mode. Reflection mode semiconductors are not suited for use as a photocathode in a compact image intensification tube, since the usage requires the emitted electrons to exit from the photocathode at an end opposite to that which the light energy first engaged the photocathode.
However, despite great effort by government and industry technical personnel, a transmission mode InGaAs photocathode could not be manufactured. Designers were not only unable to make the InGaAs layer thin enough to be effective in the transmission mode, but were also unable to make the layer supported with an optical window layer necessary for the photocathode. For a transmission mode photocathode, an active layer thickness of 1 micrometer or less is required to achieve the desired response; however, reflection mode InGaAs layers are typically formed to a thickness of approximately 10 micrometers. The thin and high crystalline quality layers required could not be produced since the InGaAs layer would not be adequately grown to a gallium-arsenide substrate used in manufacturing the semiconductor wafer structure. Moreover, the designers could not match the crystal lattice structure of the InGaAs layer with the other semiconductor layers required in a transmission mode photocathode. Due to these difficulties, most efforts to develop an InGaAs photocathode were ultimately abandoned.
Thus, it would be desirable to provide an improved photocathode structure capable of receiving wavelengths in excess of 940 nanometers. It would be further desirable to provide a photocathode structure utilizing an InGaAs active layer. It would be further desirable to provide a method of manufacturing a photocathode structure capable of responding to wavelengths in excess of 940 nanometers. It would be still further desirable to provide a method of manufacturing a photocathode structure having an InGaAs active layer.